Bone screw and method for manfacturing the same

ABSTRACT

A faceted bone screw and a method for manufacturing the same includes a screw thread configuration having facets that are substantially transverse to the thread. The facets are generally made up of a plurality of transitioning peaks and valleys which vary the depth of the thread and are disposed in one or more locations throughout the threaded portion of the bone screw. The facets operate to reduce the torque required to drive the bone screw into bone, while at the same time operate to assist in anchoring the bone screw within the bone once inserted therein, and thereby reduce the possibility for the screw backing out after insertion.

BACKGROUND

1. Technical Field

The present principles relate to orthopedic (bone) screws. Moreparticularly, it relates to a faceted orthopedic screw and method formaking the same.

2. Description of Related Art

Medical screws or Orthopedic (bone) screws or threaded pins are commonlyused in orthopedic procedures where it is required to set a bone ormultiple bones in a position that is secure with respect to either 1)the adjacent bone or bone part for which the screw is used; or 2) thesurgical splint or other external fixation device that is maintained inposition using the bone or orthopedic screw. As used herein, the term“bone screw” and/or “orthopedic screw” are interchangeably used hereinand shall include all known medical and orthopedic screws and threadedpins that are used human and/or animal bones.

One common concern in the use of bone screws is the splitting of thebone during the insertion of the screw. Splitting often occurs when theworkpiece (e.g., bone) is brittle by nature, and the friction betweenthe screw and the bone requires higher torques to sufficiently penetratethe bone for proper application.

Another concern is the potential for the screws to loosen or “back out”after installation. This loosening can result in the mis-setting of abone and require supplemental procedures to be performed to correct thesame.

It is would therefore be desirable to have a bone screw that eliminatesthese problems without requiring any change in the current approvedprocedures for the installation and withdrawal of such bone screws.

SUMMARY

The faceted bone screw of the present principles will operate to reducethe friction between the screw and the bone, thereby reducing the torquerequired to drive the screw and/or threaded pins into the bone. Thistorque reduction can thereby assist in lowering the rate of splittingosteoporotic bone particularly around the shaft of the screw while thebone screw is inserted.

The faceted bone screw of the present principles will also reduce thelikelihood of bone screws and threaded pins backing out of the bone dueto improved osteointegration between the faceted threaded portion of theimplanted device and the bone.

According to one implementation, the method of manufacturing anorthopedic screw includes loading a bar stock of material into a screwcutting machine, cutting a thread into at least a portion of the barstock with a cutting head, and forming a plurality of facets in thethread during the cutting by imparting a controlled vibratory effect onthe bar stock and a cutting head. The facets being formed by a pluralityof peaks and valleys of varying a depth of the thread for at least aportion of the same.

According to another implementation, the method of manufacturing anorthopedic screw includes loading a bar stock into a bar feeder,selecting a guide bushing having a desired clearance related harmoniclevel. The harmonic level being configured to impart a controlledmilling effect to the loaded bar stock. A circular threading tool isthen selected and installed corresponding to the desired threadconfiguration. The bar stock is then cut at the desired harmonic levelusing the circular threading tool to produce a faceted thread milledinto at least a portion of the bar stock under the controlled millingeffect of the guide bushing.

Other aspects and features of the present principles will becomeapparent from the following detailed description considered inconjunction with the accompanying drawings. It is to be understood,however, that the drawings are designed solely for purposes ofillustration and not as a definition of the limits of the presentprinciples, for which reference should be made to the appended claims.It should be further understood that the drawings are not necessarilydrawn to scale and that, unless otherwise indicated, they are merelyintended to conceptually illustrate the structures and proceduresdescribed herein.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings wherein like reference numerals denote similarcomponents throughout the views:

FIG. 1 is cross sectional view of a bone screw according to the priorart;

FIG. 2 is a cross-sectional view of a faceted bone screw according to animplementation of the present principles.

FIG. 3 a is a plan view of the bone screw cutting machine that is usedto manufacture the bone screw according to an implementation of thepresent principles;

FIG. 3 b is a plan view of the bone screw cutting machine that is usedto manufacture the bone screw according to an implementation of thepresent principles;

FIG. 3 c is a plan view of the revolving guide bush of the screw cuttingmachine used to manufacture the bone screw of the present principles;and

FIG. 4 is a flow diagram of the method for manufacturing a faceted bonescrew according to an implementation of the present principles.

DETAILED DESCRIPTION

FIG. 1 shows a cross section of a bone screw 10 according to the priorart. The shaft 12 includes a thread 14 that can extend any length of theshaft 12, including the entire length of the same. The thread generallyhas a consistent non-variable depth D depending on the particularapplication for that screw. The pitch, which relates to the distancebetween adjacent threads, is also generally consistent for most bonescrews and fasteners.

Those of ordinary skill in the art will recognize that one or moredifferent portions of the shaft 12 can include threads 14, oralternatively the entire shaft 12 can be threaded. These same conceptsapply to the bone screw of the present principles.

FIG. 2 shows a cross section of a faceted bone screw 20 according to animplementation of the present principles. The bone screw 20 has a shaft22 having threads 24 which include one or more facets 26 a, 26 b and 26c. These facets are generally transverse to the thread groove and extendacross the same for some or part of the overall thread length. Althoughshown transverse to the thread, it is contemplated that the facets maybe offset from a pure transverse relationship with the thread groove. Byincorporating facets 26 into the shaft within the thread groove 24, aplurality of peaks 28 and valleys 30 are formed therein. The facets 26are disposed at different angles α and β with respect to the nextadjacent facet. The angles α can be in a range of 90-170 degrees whilethe angles β can be in a range of 100-175 degrees. The implementation ofthe facets 26 will provide for a varying depth D of the thread.

As shown, there are several peaks 28 and valleys 30 formed by the facets26 at varying depths within the thread, each having rising/falling sidesdepending on the direction of rotation of the shaft 22. These peaks andvalleys, in conjunction with the rising/falling sides operate to reducethe friction between the bone and the screw and thereby operate toreduce the torque required to drive the bone screw into and remove froma bone. As will be appreciated, when the shaft 22 is rotated in onedirection, the rising sides of the respective peaks will graduallyoperate to penetrate the bone and once the peak is met, the frictionbetween the bone and the screw thread is substantially reduced as thebone passes over the falling side of that peak.

By repeating this process in a series like configuration throughout thethread, the overall torque required to drive the bone screw can bereduced by up to 50% (depending on the size of the screw and the bonebeing penetrated).

Once inserted into the bone, the bone will permit osteointegration withthe facets 26 (including the peaks and valleys), and the facets becomelike anchors for preventing the screw from loosening (i.e., “backingout”) after inserted by the doctor. However, when the bone screw must beextracted, a simple application of torque in the loosening directionwill cause the bone to loosen or break free from the facets 26, andfacets will once again operate to reduce the torque in required in theremoval of the bone screw.

FIG. 2 b shows another implementation of the bone screw 20 where thefacets 36 are concave in nature and the peaks are designated by thepoints 38 between the respective concave facets 36. In thisimplementation, the valleys would be considered at the base of eachconcave facet 36, and the friction reduction would be omni directional(i.e., work the same in both clockwise and counterclockwise directions).

In order to manufacture the bone screw in a reproducible, certifiablemanner, a precise manufacturing technique is employed using a Swiss typescrew machine tool. Those of ordinary skill in the art will recognizethat this time tuning (i.e., lathe) or multiple axis Swiss type CNC(Computer Numerically Controlled) screw machine is only one example ofthe type of machine that could be properly configured to manufacture thefaceted bone screw disclosed herein, and that other types of machinesmay also be implemented without departing from the spirit of the presentprinciples.

FIG. 3 a shows a plan view of a Swiss cutting machine 300 used tomanufacture the bone screw of the present principles. This is thesliding headstock type CNC automatic lathe that is generally composed ofa headstock 302, a guide bushing (or guide collet) 304, a live toolholder 306, a sub spindle 308, and a tool holder slide 310. The toolholder slide includes one or more tools or dies 311 that can be usedduring other cutting processes. Although shown here for exemplarypurposes, the present principles may not require the tool holder slide310 during the process of manufacturing the faceted bone screw.

The headstock 302 includes a main spindle 312 and a sliding unit (notshown). The main spindle 312 chucks a bar with the guide bushing 304 andprovides it with rotary motion. The sliding unit provides reciprocatingaction on the material in the Z-axis direction (longitudinal) with theCNC control. Feeding of a bar in the Z1 axis direction is provided bythe headstock during the main machining. The live tool holder 306includes a tool or cutter 307 that cuts the thread onto the (wires) barstock used to form the same.

FIG. 3 b shows a plan view of the live tool holder 306 of the Screwcutting lathe/machine 300. The live tool holder is capable ofreciprocating motion in the X-axis and Y-axis under the CNC Control, andwill feed material in a diametric direction during main machining. Thetool post makes the cutting tool contact the bar near the guide bushing304 and cooperates with the headstock 302 to execute the machining. Thetool holder (not shown), the 4-spindle sleeve holder 314 and the4-spindle cross drilling/milling unit 316 are attached to the tool post.The cutting tool will be attached to the tool holder to execute turning.

The front machining tool holder is attached to the sleeve holder 314,and executes a front drilling, tapping and boring action. Power driventools can be attached to the 4-spindle cross unit 316, providing arotating motion for drilling, tapping and end milling, etc., to performcross or front drilling, tapping and milling.

The X-axis performs a diameter direction feed of the tool holder and thetool selection of the 4-spindle cross drilling/milling unit. The Y-axisperforms the tool selection of the tool holder, tool selection of thesleeve holder 314 and a diameter direction feed of the 4-spindle crossdrilling/milling unit 316.

The guide bushing 304 supports a bar near the machining position toprevent material from bending, and thereby helps to achieve highlyaccurate and reproducible machining. In this unit, the guide bushing 304supports most of the cutting load in the diametric direction, and themachining accuracy is somewhat dependent on the clearance between theguide bushing 304 and the bar. Therefore, selection of the bar is basedon the precision required for the outer diameter of the material beingcut with the threads of the present principles. The guide bushing 304 ispreferably a revolving guide bush 320 (see FIG. 3 c) that issynchronized with the main spindle. Generally the guide bush 320 ispositioned within the guide bushing 304.

The sub spindle 313 chucks a bar with the guide bushing (collet) 304 andprovides a rotary motion. The sliding unit provides materialreciprocation in the ZB-axis direction (longitudinal) and the XB-axisdirection with the CNC control.

The tool holder 310 provides ZB-axis direction feed in the backmachining, and XB-axis direction feed in the tool selection ofsub-spindle unit 308. The various roles of the back attachment machiningcan be roughly classified as follows:

Non-pip machining: The back attachment chucks a work piece in thecutting process and performs the cutting process by synchronous rotationwith the main spindle so as to obtain a cutting-off surface withoutdowel.

Z-ZB synchronous control: The back attachment chucks a work piece at thesame time with the main spindle during the main machining. It alsoperforms a synchronous operation in direction of the Z/ZB-axis, or makesa synchronous rotation with a main spindle so that it suppresses bendingor warping of the bar.

Back machining: The live tool holder 306 performs back machining of thecutting-end surface and periphery thereof in cooperation with the backsub-spindle unit 308 of the tool post.

Sub-spindle unit 308 <This is not included in type 540S of the machine>:The tool holder 306 for machining of the cutting-end surface is attachedto the back machining sub-spindle unit 308 to perform the backsidedrilling, tapping and boring. Selecting the drive system for powerdriven attachment (this is an option) permits the attachment of apower-driven tool until and the machining of the back off-centertapping/milling.

FIG. 4 shows the method 400 for manufacturing the faceted bone screw inaccordance with a semi-automatic implementation. In accordance with onemethod of the present principles, a bar stock of desired material isloaded (402) into the bar feeder. A collet is installed (404) in thework holding axis. A custom made guide bushing, fabricated to the sizerequire to produce a desired level of clearance related harmonics, isinstalled (406) into the machine spindle axis. A circular threading toolwhich has been ground to produce the desired thread configuration isinstalled (408) one a live tool holder.

According to one aspect, the facets of the faceted bone screw areapplied through a precisely controlled vibratory effect through theapplication of clearance related harmonics during the screw cuttingprocess. Thus, by adjusting the size of the guide bushing (guide collet)we can define the clearing between the same and the bar stock. This“clearance” generates a clearance related harmonic (or a controlledvibratory effect) as the bar stock is fed through the spindle axispassing by the rotating circular threading tool which is generating thethread configuration onto the bar stock. Through the control of theclearance, the vibratory effect is accurately controlled. Examples ofsuch clearance would be 0.0002-0.005 inches.

Those of skill in the art will recognize that the Swiss type screwmachine is a computer programmable machine, and as such, theaforementioned processed can be computer controlled by the machine onceprogrammed accordingly. For example, the machine cane be programmed sothe threading tool produces the thread configuration in one pass ormultiple passes, depending on the size of the bar stock, the amount ofmaterial to be machined, and desired finish.

Other multiple features of the faceted bone screw can be performed priorto, or after, the thread configuration is generated onto the bar stock,such as screw head generation, drilling pilot details, driveconfigurations, coatings and/or any further surface preparationtreatments, etc.

Those of skill in the art will recognize that the “bar stock” referredto throughout this specification is the material which theorthopedic/bone screw is made of. Examples of this material, as they arecurrently being used are Titanium, Stainless Steel, cobalt chromium, andabsorbable biocompatible plastics. The present principles may apply toany known or not yet known material used for orthopedic/boneapplications.

It is to be understood that the present principles may be implemented invarious forms of hardware, software, firmware, special purposeprocessors, or a combination thereof. Preferably, the present principlesmay be implemented as a combination of hardware and software. Moreover,the software is preferably implemented as an application programtangibly embodied on a program storage device. The application programmay be uploaded to, and executed by, a machine comprising any suitablearchitecture. Preferably, the machine is implemented on a computerplatform having hardware such as one or more central processing units(CPU), a random access memory (RAM), and input/output (I/O)interface(s). The computer platform also includes an operating systemand microinstruction code. The various processes and functions describedherein may either be part of the microinstruction code or part of theapplication program (or a combination thereof) that is executed via theoperating system. In addition, various other peripheral devices may beconnected to the computer platform such as an additional data storagedevice and a printing device.

It is to be further understood that, because some of the constituentsystem components and method steps depicted in the accompanying Figuresare preferably implemented in software, the actual connections betweenthe system components (or the process steps) may differ depending uponthe manner in which the present principles is programmed. Given theteachings herein, one of ordinary skill in the related art will be ableto contemplate these and similar implementations or configurations ofthe present principles.

While there have been shown, described and pointed out fundamental novelfeatures of the present principles, it will be understood that variousomissions, substitutions and changes in the form and details of themethods described and devices illustrated, and in their operation, maybe made by those skilled in the art without departing from the spirit ofthe same. For example, it is expressly intended that all combinations ofthose elements and/or method steps which perform substantially the samefunction in substantially the same way to achieve the same results arewithin the scope of the present principles. Moreover, it should berecognized that structures and/or elements and/or method steps shownand/or described in connection with any disclosed form or implementationof the present principles may be incorporated in any other disclosed,described or suggested form or implementation as a general matter ofdesign choice. It is the intention, therefore, to be limited only asindicated by the scope of the claims appended hereto.

1. A method of manufacturing an orthopedic screw comprising: loading abar stock of material into a screw cutting machine; cutting a threadinto at least a portion of the bar stock with a cutting head; forming aplurality of facets in said thread during said cutting by imparting acontrolled vibratory effect on the bar stock and a cutting head, saidfacets being formed by a plurality of peaks and valleys of varying adepth of the thread for at least a portion of the same.
 2. The method ofclaim 1, wherein said controlled vibratory effect further comprisesselecting a guide bushing for the screw cutting machine having apredetermined clearance with respect to the bar stock, said clearanceproviding a desired harmonic level for generating the controlledvibratory effect.
 3. The method of claim 1, wherein said cutting furthercomprises selecting a circular cutting tool configured to a desired,reproducible, facet design using the known controlled vibratory effect.4. The method of claim 3, wherein said cutting and forming is performedin at least two dimensions around said bar stock.
 5. The method of claim2, wherein said desired harmonic level is generated by selecting aclearance between the guide collet and the bar stock in a range of0.0002 to 0.005 inches.
 6. A method of manufacturing an orthopedic screwcomprising: loading a bar stock into a bar feeder; selecting a guidebushing having a desired clearance related harmonic level, said harmoniclevel configured to impart a controlled milling effect to the loaded barstock; selecting and installing a circular threading tool previouslyconfigured to a desired thread configuration; and cutting the bar stockat the desired harmonic level using the circular threading tool toproduce a faceted thread milled into at least a portion of said barstock under the controlled milling effect of the guide bushing.
 7. Themethod of claim 6, wherein said controlled milling effect comprises acontrolled vibratory effect on the loaded bar stock generated by theclearance related harmonic level.
 8. An orthopedic screw comprising: ashaft; a thread cut into at least a portion of said shaft, said threadshaving a depth; and a plurality of facets formed in at least a portionof said thread and extending transverse thereto, said facets havingpeaks and valleys creating a varying depth of said thread and configuredto reduce friction between a bone and the threaded shaft therebyreducing the torque required to drive said screw into a bone whilesimultaneously acting to prevent undesirable backing out of the shaftafter insertion into the bone.
 9. The orthopedic screw of claim 10,further comprising surfaces between said peaks and valleys, saidsurfaces being flat.
 10. The orthopedic screw of claim 10, furthercomprising surfaces between said peaks and valleys, said surfaces beingconcave.